The invention relates to a method for the combustion of hydrocarbon-containing fuels, in particular of fuel gases. In addition, the invention also relates to the associated device.
For reasons of availability and efficiency, the operation of gas turbine burners increasingly demands the firing of fuels of different quality, which, in the most unfavorable case, may be reflected in differences in calorific value, but, even if the calorific value remains the same, leads to differences in the laminar and the turbulent flame velocity, in the induction time and in the reaction rate. The specific design of a combustion chamber and of a burner head for such burners therefore allows optimal operation with high efficiency and low emissions in each case only for an exactly specified fuel. Similarly, the load range in which a burner can be operated in a stable way and with low emission is largely defined by the respective design.
The use of electrical fields in combustion processes is known, for example, from WO 2006/067108 A1. It is proposed here to control a burner by the application of electrical fields and thereby to improve flexibility in terms of the load range and fuel quality. However, the use of electrical fields, which are not sufficient for igniting and maintaining an electrical gas discharge plasma, allows only a restricted control of combustion which is based on the utilization of the ions generated in the flame itself.
The use of plasmas for inducing pyrolysis and partial oxidation is known from WO 2006/034983 A1. In this case, it is proposed to stabilize combustion by plasmas which are generated by high-frequency excitation in the combustion space. The generation of plasmas in the combustion space of a burner by an electrode arranged in the combustion space is proposed in WO 2003/081130 A1. EP 1 512 913 A1 describes the treatment of fuel/air mixtures in the supply to the burner by cold plasmas for the control of combustion. U.S. Pat. No. 5,640,841 A describes the ignition of a turbine burner by what is known as a plasma torch which can be operated as a pilot not only for the ignition, but also for the stabilization of combustion. WO 2005/017410 A1 proposes to activate the fuel during supply to the burner by dielectrically impeded discharge and thus to improve combustion, for example, in the burners of aircraft engines. For the same purpose, WO 2004/085694 A2 describes the improvement of combustion by the treatment of a gas stream by dielectrically impeded discharges, this gas stream containing either fuel or a mixture of fuel and of an oxidant, for example air, which can be used for the combustion.
In the event that the gas discharge plasma is generated in the combustion space, the methods already known from the related art have the critical disadvantage that, in this case, harmful by-products are formed to an increased extent, such as may occur in smaller quantities even during combustion. Examples of this are, in particular, the formation of nitrogen oxides and carbon monoxide in the case of lean combustion and also the formation of soot and ammonia in the case of rich combustion. If the gas discharge plasma is operated in the fuel supply, in which there is no oxygen present, a product of this pretreatment is soot which often cannot be converted completely during combustion and is therefore also released as a harmful emission.
When the plasma is used in already mixed fuel/air mixtures, so as not to induce combustion already in the supply line to the combustion space, the fuel has to be highly overdosed, as compared with stoichiometric combustion. However, then, the products of plasma treatment, in addition to the hydrogen which promotes combustion, are still carbon monoxide, soot and ammonia. In the same way as soot, carbon monoxide is converted only slowly during combustion and may consequently increase the pollutant emission of the burner if the dwell time in the burner is insufficient. By contrast, alternatively, ammonia, in the case of lean combustion, is converted into nitrogen oxide or, in the case of rich combustion, is not oxidized or is oxidized only partially into laughing gas (N2O) and consequently likewise contributes to the emission of pollutants or greenhouse gases.
The use of plasmas for inducing steam reforming is known. FR 2 724 806 A1 discloses the steam reforming of hydrocarbons and volatile organic substances, which is induced by a gliding arc plasma which avoids some of the problems described: with suitable management, steam reforming can avoid the formation of soot, on the one hand, and the formation of nitrogen oxides, on the other hand.
Disadvantages of the abovementioned methods and devices for steam reforming by plasmas for use in combustion control are that a large reactor volume is required for the reaction of the radicals, that is to say chemically active molecules or atoms, and ions which a plasma of sufficiently high power density provides. Furthermore, plasmas of high power density have been achieved in gases at pressures of one atmosphere and above hitherto only by gas discharges which are in direct electrode contact, so that electrode-guided gas discharges are obtained.
An erosion of the electrodes by the arc which is then unavoidably formed is reduced by measures which force a rapid movement of the arc plasma, including its foot points, on the electrodes. Usually, this is achieved by a rapid gas flow, as described in FR 2 724 806 A1. However, should the gas flow be insufficient, the power fed locally into the electrodes may become very high, thus entailing the explosive fusion and evaporation of large quantities of the electrode material. Furthermore, there is no indication of how steam reforming can be coupled efficiently to combustion processes.